1. Field of the Invention
The present invention relates to a radiation imaging (radiographic) apparatus for forming a three-dimensional image by using a cone beam from a radiation generating unit, and more particularly, to a radiation imaging apparatus in which a distance between a radiation generating unit and a two-dimensional detector is appropriately selected.
2. Related Background Art
In recent years, development of a two-dimensional detector for radiation imaging (also referred to as an FPD (Flat Panel Detector)) has been advanced to acquire digital data of a large-sized image. Japanese Patent Application Laid-Open No. H09-288184 discloses such an FPD. Particularly, for the purposes of a so-called radiography, an imaging apparatus using a two-dimensional detector with a large light receiving surface of 43 (cm)×43 (cm) has been put into a practical use.
On the other hand, an X-ray CT apparatus is known, in which an X-ray detector detects X-rays passing through a subject exposed to the X-rays, and a transmission image (a so-called scanogram or SCOUT image), a tomographic image or a three-dimensional image of the subject is acquired based on an X-ray detection output (the number of photons of the X-rays) from the detector.
In connection with the X-ray CT apparatus for forming a three-dimensional image, a cone-beam CT apparatus has been developed owing to improvement of developed technique of a two-dimensional detector, and so forth. While an X-ray beam is thinly sliced in a Z-direction (this sliced beam is called a fan beam) in a typical X-ray CT apparatus, an X-ray beam expanding also in the Z-direction (referred to as a cone beam) is used, and the cone beam is detected by the two-dimensional detector in the cone-beam CT (referred to as a CBCT). Compared with the CT using the fan beam, the cone-beam CT has a wider range over which the subject can be imaged by a single scanning rotation. The cone-beam CT has accordingly advantages that the number of rotations can be reduced, and imaging efficiency can be increased. In other words, an increase in the imaging efficiency can be attained by widening a cone angle which is a spread of the cone beam. On the other hand, however, in the event that the cone angle is widened too largely, a reconstruction error is liable to occur in a reconstruction image.
With respect to the cone beam CT, there exists a type in which a scan (collection of projection data) is executed while a pair of an X-ray generating source and a detector are rotated around the subject. Japanese Patent Application Laid-Open No. H10-21372 discloses such a type. However, in a case of cone angle below a predetermined angle, a distance between the two-dimensional detector and the radiation generating source must exceed a predetermined value such that all the light receiving surface of the two-dimensional detector with a large light receiving surface can be irradiated with the radiation from the radiation generating source. Accordingly, in the CBCT of the type in which the scan (collection of projection data) is executed while a pair of the X-ray generating source and the detector are rotated around the subject, it is difficult due to a size of the CBCT apparatus to set the distance between the two-dimensional detector and the radiation generating source such that a large light receiving surface of the two-dimensional detector can be effectively used.
On the other hand, development has been advanced in connection with a practical use of the CBCT of a subject rotating type in which a pair of an X-ray generating source and a detector are fixed, and instead the subject is rotated (without a helical movement). For example, Japanese Patent Application Laid-Open No. 2000-217810 discloses a CBCT of such a type.
Further, there is a background of the X-ray radiography that the distance between the subject and the radiation generating source has been sought in a try-and-error manner in its tradition of about a hundred years. In such a background, one criterion of a good X-ray technologist is an ability of appropriately determining the distance between the subject and the radiation generating source. It is thus important in the radiation imaging apparatus to appropriately determine the distance between the two-dimensional detector and the radiation generating source. Hence, to put into a practical use the CBCT of the subject rotating type in which the subject is rotated, and for which the X-ray technologist has no CT scanning experience, it is considered specially critical to appropriately determine the distance between the two-dimensional detector and the radiation generating source.
Furthermore, in the CBCT disclosed, for example, in the above-referenced Japanese Patent Application Laid-Open No. 2000-217810, a distance between a focal position of an X-ray generating apparatus and an input surface of an X-ray image intensifier is set to 1200 mm, a distance between the focal position of the X-ray generating apparatus and a rotational center of a rotating apparatus, i.e., a rotational radius r of the X-ray generating source, is set to 800 mm, and accordingly a visual field of a transmission X-ray image has a spherical shape with a diameter of about 260 mm (260 mmΦ) in a case of the X-ray image intensifier of a 16-inch type (the size of an image plane in a horizontal direction is 400 mm). A cone angle calculated under those conditions is 9.5 degrees on one side, and totally 19 degrees. It is, however, likely that a reconstruction error of the reconstruction image occurs in a peripheral region of the-visual field (mainly a peripheral region of the two-dimensional detector), and no appropriate cone angle has not yet been acquired.
Incidentally, in the CBCT of the helical or spiral type (disclosed in the above-referenced Japanese Patent Application Laid-Open No. 10-21372, for example), its cone angle is set to a relatively small value (from one degree to two degrees), and data of an overall object area is collected using a plurality of rotations. Thus, no study of a range of the cone angle, in which the reconstruction error is liable to occur, has been performed in the CBCT of the spiral type.
In the CBCT of the subject rotating type in which a pair of the X-ray generating source and the detector are fixed, and instead the subject is rotated, however, there exists the problem that the distance between the two-dimensional detector and the radiation generating source is not yet appropriately determined. Further, in the CBCT, the reconstruction error of the reconstruction image is likely to occur, and no appropriate cone angle is established.
As discussed in the foregoing, it is conventionally difficult to appropriately determine the distance between the two-dimensional detector and the radiation generating source, and a variety of measures for solving such a problem have been therefore desired.